1. Field of the Invention
The present invention relates to power converters, and more particularly, to the switching power converters and switching regulators.
2. Description of Related Art
Power converters have been used to convert an unregulated power source to a regulated voltage and/or current source. The transformer of the power converter is equipped to provide the isolation between the input power source and the application apparatus for the safety purpose. FIG. 1 shows a traditional switching power converter. A transistor 5 is connected to the primary winding NP of a transformer 40 for switching the transformer 40. The secondary winding NS of the transformer 40 thus generates a power source coupled to the output of the switching power converter through a rectifier-and-filter 50. Due to the geometrical limitation of the transformer, a leakage inductor 45 will be generated in the transformer 40. The inductance LL of the leakage inductor 45 means the energy that cannot be transferred to the secondary winding NS of the transformer 40. A worse coupling between the primary winding and the secondary winding will cause a higher leakage inductance of the transformer. Therefore, a smaller transformer generally includes a higher leakage inductance. The power PL produced by the leakage inductor 45 can be expressed as,
                              P          L                =                              1            2                    ×                      L            L                    ×                      I                          M              2                                ×                      F            S                                              (        1        )            wherein the IM is the switching current of the transformer 40; the FS is the switching frequency for the switching power converter.
A diode 70, a resistor 71 and a capacitor 72 form a snubber circuit and consume the power caused by the leakage inductor 45. Otherwise, the energy of the leakage inductor 45 will become a surge voltage to stress the transistor 5.
Equation 1 shows the power loss of the leakage inductance increased in response to the increase of the switching frequency. Therefore, how to save the power of the leakage inductance becomes a major concern of power saving, especially for the switching power converter including a small transformer and high switching frequency.